This invention relates generally to the field of superconducting electronics, and more particularly, to a transimpedance amplifier for a weak current source, which may originate from superconducting electronics, to interconnect with conventional semiconductor or other high impedance electronics.
Superconducting electronics yield important advantages such as the high speed, minimal noise, and low power not available with conventional semiconductor electronics. Semiconductor electronics, on the other hand, have the unparalleled advantages of memory capabilities and a well-developed technology base. A hybrid technology to exploit the advantages of both superconductors and semiconductors is developing. One of the most challenging aspects facing developers of this hybrid technology is the development of an interface circuit to convert signals from superconducting circuits to semiconductor technologies, such as CMOS. See Ghoshal et al., Spice Models and Applications of Superconducting FETS and Higher-Voltage Josephson Gates, 1991 IEDM Conference.
Such transimpedance amplifiers using conventional semiconductor technology requires exceedingly complicated circuits having a number of transistors and many passive components which perform within a limited temperature range and which are characterized by high noise. Typical pre-amplifiers used may consist of several MOSFET stages as discussed in Paik et al., "A Staring Monolithic FPA with High Speed Readout and Frame Averaging," Northrup Technical Report, p. 11 (December 1987). MOS devices, however, generally cannot operate at low superconducting temperatures, and performance is not uniform across the temperature ranges of interest. This limitation is in fact true for all semiconductor-based systems because of carrier freeze-out and other thermal effects. To data, there is no superconducting device which will transform a weak current from a superconducting source to a higher voltage for use in semiconductor circuits, and which allow for signal conversion from a low impedance circuit to a high impedance system.
It is thus an object of the invention to provide impedance conversion from a low impedance circuit, which may or may not include superconducting electronics, to a higher impedance circuit, typically conventional semiconductor electronics. This object is achieved through the use of the superconducting flux flow transistor (SFFT), and given that the impedance characteristics of the differential structure of the transimpedance amplifier of the invention is not significantly different from the impedance characteristics of the SFFT itself. An additional advantage of this particular feature of the invention results in low power dissipation.
It is another object of the invention to provide for amplification of a weak current to sufficient voltage suitable for semiconductor applications. This differential amplifier configuration of SFFTs increases the output voltage available. An advantage of this increased output voltage is increased compatibility with multiple forms of conventional electronics.
It is another object of the invention to provide for the reduction of noise which is achieved by the differential configuration of SFFTs which allows the circuit to be used in more electromagnetically sensitive environments.
It is another object of the invention to provide for wide bandwidth from GHz down to DC with adequate gain. Bandwidth of the amplifier itself is preserved by the differential configuration of the transimpedance amplifier.
These and other objects are achieved by the invention specified and claimed herein as a transimpedance amplifier which has at least a first and a second SFFT connected in parallel, each having a control line and means to apply a signal to the control lines of each SFFT, but the signal as applied to one SFFT being of opposite polarity than the signal as applied to the other SFFT, and a means to provide a current bias to each SFFT sufficient to drive each SFFT transistors into a flux flow state, wherein the signal applied to the control lines of each SFFT is converted to higher signal and higher impedance levels which is taken across the means to provide a current bias to said transistors. In addition, impedance elements having a resistive component can be connected between each SFFT and the means to provide a current bias to each SFFT wherein the impedance elements increase said output signal and impedance.
A double differential transimpedance amplifier then comprises a first and a second SFFT connected in parallel, each having a current bias applied, and a control line input of low impedance and weak current, wherein the input as applied to the first SFFT is of opposite polarity than that applied to the second SFFT, and an output of the first and second SFFTs taken across the bias wherein the output is an amplified signal of the low impedance, weak current input; and further comprising a third and a fourth SFFT connected in parallel, the third and fourth SFFT also having a current bias applied, and an ancillary impedance connected between the bias and each of the third and fourth SFFT, and having a second input which is the output of the first and second SFFT, wherein the input as applied to the fourth SFFT is of opposite polarity than that applied to the third SFFT, wherein the output of the third and fourth SFFT is an amplified signal of the second input, and the bias is sufficient to drive each SFFT into a flux flow state.
It is envisioned that the input to the transimpedance amplifier can be from any weak current source, and can be high temperature superconductor electronics, superconductor electronics, or conventional semiconductors. The invention is particularly useful when the input is provided by far-IR focal plane detectors or Josephson junctions because of the temperature and frequency range enabled by the invention.
The invention, moreover, comprises a differential amplifier stage wherein the input to the first amplifier stage is derived from the low impedance signal source, and the output of each stage is applied as input to the control lines of the next amplifier stage, with the final output differential amplifier stage having the resistive elements between the bias and the SFFTs for appropriate impedance matching to the processing electronics, usually of conventional semiconductor electronics.
The invention is described with reference to the following figures.